Introducing a new 7-ring fused diindenone-dithieno[3,2-b:2',3'-d]thiophene unit as a promising component for organic semiconductor materials

A novel π-conjugated molecule, EtH-T-DI-DTT is reported, which is fused, rigid, and planar, featuring the electron-rich dithieno[3,2-b:2’,3’-d]thiophene (DTT) unit in the core of the structure. Adjacent to the electron-donating DTT core, there are indenone units with electron-withdrawing keto groups. To enable solubility in common organic solvents, the fused system is flanked by ethylhexylthiophene groups. The material is a dark, amorphous solid with an onset of absorption at 638 nm in CH2Cl2 solution, which corresponds to an optical gap of 1.94 eV. In films, the absorption onset wavelength is at 701 nm, which corresponds to 1.77 eV. An ionisation energy of 5.5 eV and an electron affinity of 3.3 eV were estimated by cyclic voltammetry measurements. We have applied this new molecule in organic field effect transistors. The material exhibited a p-type mobility up to 1.33 × 10−4 cm2 V−1 s−1.

S2 the solutions with a substrate concentration of ca. 10 −3 to 10 −4 mol L −1 , using tetrabutylammonium hexafluorophosphate as electrolyte. The solutions were degassed with argon prior to the measurements. As a reference, the ferrocene redox couple Fc/Fc + was applied. As electrodes, a three-electrode setup was taken with platinum disk as working, platinum wire as counter, and silver wire as pseudo-reference electrodes [5]. As energy levels below vacuum for the ferrocene/ferrocenium (Fc/Fc + ) redox couple, 4.8 eV is taken [6].
Spectroelectrochemical measurements were performed by linking cyclic voltammetry (CH Instruments 660E Electrochemical workstation) and UV-vis (Shimadzu UV-2600 Spectrophotometer in the range 200-1200 nm) together. The experiment was performed using a solution-state spectroelectrochemistry kit, where the quartz cuvette path length is 1 mm. The electrodes used were platinum mesh, platinum wire, and silver wire as working electrode, counter electrode and reference electrode, respectively.
The electrodes were placed in the solution (0.1 mM monomer and 0.1 M electrolyte dissolved in dry DCM). In the oxidation process, the potential was initially increased gradually from 0 V to 1.5 V by measuring the absorption every 0.1 V. Then, the potential was decreased to zero gradually and absorption was measured. In the reduction process, the potential was decreased gradually from 0 V to −1.6 V then was increased gradually from −1.6 V to 0 V and the absorption was measured every 0.1 V.
For OFETs, wafers, in which 230 ± 10 nm thick SiO2 is the dielectric layer, and 30 nm Au + 10 nm indium tin oxide (ITO) is the adhesion layer as source/drain electrodes, and n-doped silicon is the gate, were purchased from Fraunhofer Institute for Photonic Microsystems, product No. 1301.

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Each chip accommodates 16 transistors, each four with L = 2.5 μm, L = 5 μm, L = 10 μm, and L = 20 μm [9]. The width W is always 1 cm. Solutions of EtH-T-DI-DTT in chloroform and chlorobenzene were stirred overnight before coating. The chips were washed with deionised water, isopropanol, and acetone. If SAMs were used, octadecyltrichlorosilane (OTS) from toluene solution, or pentafluorobenzenethiol (PFBT) from ethanol solution, were drop-casted onto the chip. After drying with compressed air, the chip was brought into the glovebox. In the glovebox, the solution of EtH-T-DI-DTT was spin-coated with 1000 rpm onto the chip and, in some cases, annealed before testing.

Dithieno[3,2-b:2',3'-d]thiophene-2,6-diylboronic acid (25)
Two 500 mL three-necked flasks, one equipped with a stirring bar for the reaction (flask 1) and one for the SPS, were heated in the oven overnight and evacuated and refilled with argon three times. Flask 1 was charged with 4.02 g 24 (11.36 mmol, 1 equiv). The flask was evacuated and refilled with argon three times, and 200 mL anhydrous THF was injected, followed by one cycle of freeze-degassing. Over the course of 30 minutes, 10 mL n-butyllithium (2.5 M/hexane, 25 mmol, 2.2 equiv) was added at −90 °C, and the mixture was stirred at this temperature for another 20 minutes. The mixture was allowed to warm to −80 °C and stirred at this temperature for 20 minutes. The mixture was cooled to −90 °C and was stirred for 20 min at this temperature.
Then, 16 mL triisopropylborate (69.3 mmol, 6.1 equiv) was injected rapidly in one portion. The mixture was allowed to warm to room temperature overnight, and 5 mL deionised water was injected. After stirring for 15 minutes, the mixture was poured onto 1 litre deionised water.
Dropwise addition of 1 M hydrochloric acid until pH < 3 resulted in a grey precipitate to form. After extraction with diethyl ether (3 × 200 mL), the organic phases were combined. The solvent was evaporated, and the grey solid obtained was dried on a petri dish on air overnight, and dissolved in a small amount THF. After re-precipitation in petroleum ether, the obtained solid was filtered off and dried on air. Further drying in the desiccator led to 3.12 g (11.0 mmol, Y = 97%) of 25, which was obtained as grey solid.
After allowing to cool to rt, the solid was filtered and dried on air. After further drying in the desiccator, 5.21 g (11.6 mmol, Y = 79%) of 26 was isolated as grey solid.

6,6'-(Dithieno[3,2-b:2',3'-d]thiophene-2,6-diyl)bis(3-bromobenzoic acid) (28)
A 250 mL flask with a stirring bar and an attached condenser was charged with 0.53 g (0.85 mmol, 1 equiv) 27, and was evacuated and refilled with argon three times. SPS-THF (20 mL) was injected, followed by one cycle freeze-degassing. Then, a lithium hydroxide solution (0.13 LiOH, 5.33 mmol, 6.3 equiv in 35 mL deionised water), which had been freeze-degassed once, was S7 injected into the flask. The mixture was heated to 70 °C, and stirred at this temperature for 24.5 hours, then allowed to cool to room temperature. The THF was evaporated, and 10 mL 1 M hydrochloric acid was added, leading to a yellow precipitate, which was filtered off and washed with 200 mL deionised water, 100 mL chloroform, and dried on air. After further drying in the desiccator, 0.50 g (0.83 mmol, Y = 98%) of 28 were obtained as yellow solid.

Diindenone-DTT (29)
Intermediate 28 was dried in the desiccator overnight, aluminium trichloride was resublimed at 180 °C. A 250 mL three-necked flask with a stirring bar (flask 1), a condenser, a cooling trap, and a 250 mL two-necked flask (for SPS) were heated in the oven overnight. The three-necked flask was connected with the condenser, which was connected to a Schlenk-line via the three-way-tap 1, which other opening was connected to an additional oil bubbler. One of the outer necks of the three-necked flask was connected with the cooling trap via the three-way-tap 2. The cooling trap was also connected to the Schlenk line. The following picture shows the apparatus: After the apparatus was evacuated and refilled with argon three times, flask 1 was charged with 0.49 g (0.82 mmol, 1 equiv) 28.
The septum was removed, and 6.31 g pinacol (53.36 mmol, 5.2 equiv) were added in one portion, and the flask was closed again. Since the viscosity increased strongly after a few hours, 50 mL THF were added. The mixture was stirred overnight, and was poured into 500 mL deionised water.
Extraction was done with diethyl ether (3 × 100 mL

Intermediate 33
A 250 mL two-necked flask with a stirring bar was attached to a condenser. After addition of 0.55 g of 27 (0.89 mmol, 1 equiv), the flask was evacuated and refilled with argon three times. Then, compound 32 (1.16 g, 3.43 mmol, 3.9 equiv) and 90 mL SPS-THF were added, followed by one cycle of freeze-degassing. Afterwards, 0.20 g tetrakis(triphenylphosphine)palladium(0) (0.18 mmol, 0.20 equiv) was added, followed by two more cycles freeze-degassing. After injection of 10 mL of a 3 × freeze-degassed, aqueous K2CO3 solution (1.29 g, 9.36 mmol, 10.6 equiv), the mixture was heated to 70 °C and stirred at this temperature for 45 h. After cooling to room temperature, the aqueous phase was removed with a separation funnel. The organic solvent was evaporated, and the residue was purified by column chromatography six times (petroleum ether/dichloromethane 1:1). Removal of solvent and drying on a petri dish led to 0.60 g (0.68 mmol, Y = 76%) of the product, a yellow resin.

EtH-T-DI-DTT (1)
Aluminium trichloride was resublimed at 250 °C and was stored in a dried, Ar-filled glass ampoule.
Diacid 34 was dried in the desiccator overnight.
A 250 mL three-necked flask with stirring bar, a condenser, a cooling trap, and a 250 mL twonecked flask for solvent were heated in the oven overnight, and attached according to Figure S1.

Tetrakis(triphenylphosphine)palladium(0)[11]
Three 250 mL two-necked flasks, one with stirring bar and one for solvent, a 100 mL one-necked flask and a filter-column were heated in the oven overnight. The 250 mL flask with stirring bar and one 250 mL flask without stirring bar were connected to the filter-column, and the glassware was evacuated and refilled with argon three times. The 250 mL flask with stirring bar was charged with 1.02 g palladium dichloride (5.76 mmol, 1 equiv) and 7.55 g triphenylphosphine (28.8 mmol, 5.0 equiv). The flask was again evacuated and refilled with argon three times, and 80 mL anhydrous dimethylsulfoxide was injected.

Oxidation process
By increasing the potential gradually from 0 V to +1 V, UV-vis spectroscopy showed an initial absorption profile with λmax at 361 nm and a shallower band at 540 nm. Upon increasing the potential to +1.1 V (the first oxidation peak potential), the high energy absorption band reduced intensity and the low energy band was red-shifted to 550 nm. From +1.3 V (the second oxidation peak) to +1.5 V the low energy absorption extended into the near-IR.

Reduction process
The reduction processes were observed in the region 0 V to −1.6 V ( Figure S21). UV-vis spectra revealed a decrease in intensity with a decrease in the potential with a maximum absorption for the highest energy band at around 341 nm. The low energy band started to disappear at −1.2 V with a new, broader band emerging into the near-IR.